A number of processes exist for producing polymers of varying molecular weight. One example of a polymerization process that can be used in the production of, inter alia, olefin based polymers (e.g., polyethylene and polypropylene) is continuous solution based polymerization.
Continuous polymerization processing can be economically advantageous over batch polymerization for high-volume polymer production. As opposed to batch polymerization, continuous processing requires smaller equipment (e.g., feed tanks and pumps) per unit of throughput, and therefore is less capital intensive with reduced power consumption and lower total running costs.
Continuous solution polymerization generally involves the addition of a catalyst to a monomer and optionally a solvent mixture. Examples of suitable catalysts for continuous solution polymerization processes include, but are not limited to, single site transition metal catalysts, such as metallocene catalysts. Upon reaction, the formed polymer is dissolved in the polymerization medium or solvent, often along with any catalyst and unreacted monomer, frequently with the solution exiting the reactor having a relatively low polymer concentration, such as from about 3 wt. % to 30 wt. %. The product mixture is then passed to polymer concentration and finishing stages to separate the solvent and unreacted monomer from the mixture such that the desired polymer can be recovered in a usable form. The separated solvent and monomer can then later be recycled back to the reactor for re-use.
It is well known that polymer solutions can undergo phase separation at the lower critical solution temperature (LCST), where both the polymer-rich phase and polymer-lean phase are either substantially liquids or supercritical fluids. This phase separation is encouraged by higher temperatures and/or lower pressures, but where the pressure is still high enough to prevent formation of a vapor phase. Accordingly, it is recognized to take advantage of this phenomenon of LCST phase separation to assist in separating the solvent and unreacted monomer from the polymer product of solution polymerization processes. The first step in this separation method involves heating the product mixture under high pressure, followed by reducing the pressure to a point where two phases (polymer-rich phase and polymer-lean phase) are formed. Of the two phases that are formed, the polymer-lean phase is rich in solvent and contains most of the unreacted monomer and contains very little polymer, whereas the rich phase is polymer rich. The denser polymer-rich phase settles to the bottom of the vessel where it is pressure fed to downstream equipment where the remaining solvent is removed. The solvent-rich phase (polymer-lean phase) overflows out the top of the separation vessel where it is cooled and recycled back to the reactor for re-use. With most polymers, the polymer-lean phase contains an extremely low polymer concentration, in an amount that does not interfere with the functioning of the solvent recycle stream.
Generally, LCST separation is the preferred method of polymer recovery for most conventional continuous solution polymerization processes, as it usually is the most energy efficient process. The benefits of the LCST separation is explained in more detail in U.S. Pat. Nos. 6,881,800 and 7,163,989 both of which are incorporated by reference. However, when producing low-molecular weight polymers, for example polymers having a significant fraction of their molecular weight distribution below 10,000 g/mol, LCST may be unable to provide a clean separation between the polymer product and the solvent. Because of this, a significant fraction of the polymer product is carried overhead in the lean phase, where it can plate out and detrimentally foul equipment in the recycle solvent stream. As a result, recovery of low molecular weight polymers is generally achieved by flash vaporization of the solvent. Flash vaporization prevents the low-molecular weight polymer from being carried over in the vapor stream into the recycle solvent stream which, in turn, prevents fouling. As an example, a description of a flash vaporization separation process can be found in U.S. Pat. No. 6,204,344.
Operating conditions for flash separation and LCST operation differ primarily in the pressure under which the separation is effected (higher pressures for LCST and lower pressures for flash vaporization), which also results in lower temperatures for flash vaporization due to the greater enthalpy of a vapor phase relative to a liquid or super critical fluid (SCF) polymer-lean phase. In practice this means that production of high and low molecular weight polymers may require the use of either different polymerization plants or different separation systems. The present invention seeks to provide a flexible process and production plant that can effectively accommodate the processing of both low molecular weight and high-molecular weight polymers in the same plant and using the same separation system.